hwmon: (ads1015) Handle negative conversion values correctly

Make the divisor signed as DIV_ROUND_CLOSEST is undefined for negative
dividends when the divisor is unsigned.

Signed-off-by: Peter Rosin <peda@axentia.se>
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>